The conventional teaching in foundation and related support structures for onshore, large-scale wind turbines instructs on-site pouring of a large, thick, horizontal, heavily reinforced cast-concrete base and a vertical cast pedestal installed over the base. Such structures are referred to as a gravity foundation or a spread foundation.
The amount of cast concrete used in conventional wind turbine erection is staggering. In the United States alone, the wind industry buries in excess of 8 billion pounds of concrete each year as foundation structure (spread foundations) for approximately 4,000 wind turbines. This equates to roughly 2 million pounds of concrete poured for every wind turbine. And, once placed, this enormous mass of concrete cannot feasibly be removed, thus, forever remaining a part of the land, and removing about 300 acres of prime farm soil per year.
The conventional teaching of spread-footing, cast-in-place foundations also require a massive mobilization of resources to form, place steel rebar and pour concrete. This requires several months for construction and subsequent concrete curing prior to the tower erection work. This results in cost increases and inefficiencies in the construction and erection process. Further, the process of pouring such a massive concrete structure requires diligence to avoid improper curing and is replete with potential problems including the sophisticated planning and coordination required to pour large amounts of concrete per footing, in one continuous pour, without having cold joints. The volume of required concrete creates logistical problems as such a pour requires coordinating with multiple local batch plants the delivery plan of the large number of concrete trucks to the job site in a timely and organized manner. A further problem is the complexity of installing the rebar assembly into the foundation which requires assembling two layers of steel reinforcing meshes that are two to six feet apart across the full area of the foundation, while maintaining a strict geometric layout and specific spacing. This rebar assembly is made of extremely long and heavy rebar which requires the use of a crane in addition to multiple workers to install all the components of the assembly. The rebar often exceeds forty feet in length, thus requiring special oversized shipments that are very expensive and usually require special permits. The installation of the rebar is a labor intensive and time-consuming task requiring a large number of well-trained workers.
Another impediment of conventional foundations includes that the construction process consists of field-work which can easily be compromised by weather conditions and other site conditions, thus, weather adversely affects the foundation of such a large-scale pour of concrete.
Another problem is thermal cracking of concrete due to overheating of the concrete mass during curing. When concrete is cast in massive sections, the temperature can reach high levels and the risk of thermal cracking becomes very high. Thermal cracking compromises the structural integrity of foundations.
The current state of the art includes the teachings of the following references, which are hereby incorporated by reference for all purposes as if fully set out herein:
U.S. Pat. No. 8,661,752;
U.S. Pat. No. 8,833,004;
U.S. Pat. No. 8,458,970;
U.S. Pat. No. 5,586,417;
United States Published Application No. US 2012/0167499;
United States Published Application No. US 2014/0260023;
United States Published Application No. US 2014/0033628;
European Patent No. EP 2256338;
Published PCT patent application No. WO 2011/030199;
U.S. Pat. No. 7,618,217 B2 (Post-tension pile anchor foundation and method therefor); and
CA 2844373 A1 (Perimeter pile anchor foundation).
Despite the current state of the art, there remains a need for a viable foundation for wind turbines and similarly tall, narrow structures that reduces installation time, increases foundational stability, reduces defects during construction, minimizes erection time for the structure, and is otherwise better engineered for the specific site requirements.